Certain embodiments of the invention relate to electrical tomography, and may utilise electrode arrangements (e.g. electrode arrays) known in the art. Certain embodiments may also be used for electrical tomography applications known in the art.
Known electrical tomography techniques include electrical impedance tomography (EIT), electrical resistance tomography (ERT), and electrical capacitance tomography (ECT). EIT is known for use as a medical imaging technique in which an image of the conductivity or permittivity are part of the body is inferred from surface electrical measurements. Conducting electrodes are typically attached to the skin of the subject and small alternating currents are applied to some or all of the electrodes. Resulting electrical potentials (i.e. between electrodes) are measured, and the process may be repeated for different configurations of applied current. Investigated processing techniques are used to generate an image of the body or other medium or sample under test, from the current and voltage signals. Applications of electrical impedance tomography are not limited to medical imaging, and indeed embodiments of the present invention may be used in electrical impedance tomography techniques for evaluating any suitable medium or structure (which we may also describe as a sample). ERT also finds a wide variety of applications, and is known for use in geophysics. In this particular application, electrodes may be placed on the surface of the earth or at some other location to locate resistivity anomalies. ERT also finds application in industrial process monitoring, where arrays of electrodes may be used to monitor mixtures of conductive fluids in vessels or pipes. ERT techniques may be used in industrial process imaging for imaging conductive fluids. In this particular context, the technique may be called electrical resistance tomography. In such applications, in which embodiments of the invention may be used, metal electrodes may be placed in direct contact with the fluid (which may be a single fluid or a mixture) to be evaluated.
Electrical capacitance tomography (ECT) is a method for determination of the dielectric permittivity distribution in the interior of an object from external capacitance measurements. ECT is closely related to EIT and may also be used as a method for industrial process monitoring. Potential applications include the measurement of flow of fluids in pipes, and measurement of the concentration of one fluid in another, or the distribution of a solid in a fluid. In ECT, the measurement electrodes may take the form of metallic plates and these should be arranged so as to be sufficiently large to give a measurable change in capacitance. This means that very few electrodes are used and 8 or 12 electrodes are common.
ERT systems typically use a current source to drive an alternating current through a medium to be evaluated. However, problems arise when the medium to be evaluated is a fluid having a relatively high conductivity. Relatively high conductivity means that the equivalent electrical impedance of fluid is low, which means that, for a given current driven by a current source through the medium, corresponding response voltages from electrodes are small. Clearly, output currents from any given current source are limited, and hence measurement of the small voltages developed in or across the medium may be difficult due to the presence of noise. In other words, a low signal to noise ratio is a problem associated with ERT techniques used to evaluate high conductivity fluids. The presence of a common mode voltage is a further problem associated with attempts to use ERT techniques for evaluating high conductivity fluids. A further problem is the handling of large dynamic ranges in voltage signals from measurement electrodes to be digitised (for example by an ADC) so that they can be subsequently processed.